In Sunshine and In Shadow by Josh Lanyon

Keir and Rick are cops. Partners, best friends, and -- for a brief time -- lovers. But Rick thinks its too risky allowing yourself to fall in love with your partner, and Keir has had to accept that. Accept it and move on. In fact, he's traveling all the way to Ireland on his first solo vacation in years. Rick may just find out that green is the loneliest number.